Multi-tier storage—the use of virtual or physical storage devices with different I/O performance, data availability, and relative cost characteristics to provide differentiated online storage for computer systems—is implemented by many enterprises to optimize storage and I/O cost and performance. A traditional approach to deriving value from multiple tiers of storage is to place files on the appropriate type of storage device. More critical files are typically placed on higher-performing, more reliable (and therefore more expensive) devices; less critical files may be placed on less costly devices.
For traditional backup systems, matching a file to the most efficient storage device may seem straightforward. For example, administrators may assign files to the most efficient type of storage. However, administrators cannot effectively assign large numbers of files in this way. Thus, some multi-tier storage systems implement automatic file relocation policies that are based on access patterns of files. Unfortunately, using access patterns to define relocation policies may not result in efficient relocation in some circumstances.